Balanced granular microphone



Feb. 10,4 1948. 1 R. BuRRoUGl-ls BALANCED GRANULAR MICROPHONE Filed Sept. 20, 1943 INVENTOR.

BY m /QMQ Patented Feb. 10,A 1948 BALANCED GRANULAR MICROPHONE Louis R. Burroughs, South Bend, Ind., assignor to Electro-Voice Manufacturing Co., Inc., South Bend, Ind., a corporation of Indiana Application September 20, 1943, Serial No. 503,093

8 Claims. l

This invention relates to improvements in microphones, and more particularly to microphones of the carbon button type.

Carbon button microphones are frequently subject to harmonic distortion. Thus the diaphragm of the microphone, by virtue of its relation to the carbon granules of the microphone at one face thereof, is subject to unequal and unbalanced pressures and to an unbalanced vibrationdamping action. This imbalance prevents true response of the diaphragm to sounds impinging thereon, and distortion results. It is the primary object of this invention to reduce such distortion and obtain an improved response characteristic in a microphone of this type.

A further object is to provide a microphone with means for balancing the diaphragm thereof with respect to pressures and vibration-damping iniiuences thereon.

A further object is to provide means for damping the vibrations of a microphone to limit said vibrations to those imparted thereto by impinge` ment of sound thereagainst.V

Other objects will be apparent from the description, drawing and appended claims.

Fig. 1 isan axially transverse sectional view of a microphone, taken on line I-I of Fig. 2.

Fig. 2 is a sectional view taken on line 2 2 of` Fig. 1.

. Referring to the drawing, which illustrates one embodiment of the invention, the numeral I designates the front part and the numeral II designates the rear part of a two-part microphone housing having a substantially 'central parting plane. Parts III and II are preferably molded from plastic material to substantially` cup-shaped form withvtheir edges in engagementY to define a chamber I2 therebetween. The margins of the housing parts are secured together in face engagement by screws I 3, or the like. The

part I0 has a plurality of openings I4 formedA therein for passage of sound therethrough and into chamber I2. Part II has a central projection I5 accommodating the formation of a' tapered recess or pocket therein communicatingwith said chamber.

Each of the housing parts has a ring I6 fitting. therein, and the margin of the diaphragm I1 is` 2 illustrated tongue I8 may be imbedded in part II during the molding of said part to provide a preassembled diaphragm and housing unit. Diaphragm I1 has a frusto-conical off-set portion 20 formed centrally therein. The pocket formed in projection I5 of housing part II mounts a cupshaped truste-conical metal lining plate or button 2I with which electrical connection is made by a conductor 22 projecting through the housing Wall to provide a second microphone terminal 23. The central diaphragm portion 20 preferably projects into and is arranged coaxially with button 2|, and said parts are so shaped and related that the spacing therebetween is substantially uniform at all points. Carbon granules 24, or granules of other suitable conductive material, fill the space between said diaphragm and button, and are conned therein by a suitable retainer ring 25 spanning the space between the dia'- phragm and the housing wall and positioned concentrically relative to diaphragm projection 20.- Retainer ring 25 may be made of any suitable material having requisite properties of compressibility and resilience to permit vibration of the diaphragm responsive to sounds impinging thereagainst. Ring 25 is of a size to provide a substantial free space between the same and ring I6,

A thin sheet 26 of rubber or flexible plastic ma terial is preferably positioned against the inner face of the apertured wall of front housing part Il) and is secured thereto at its margin, as by clamping thereof between said wall and the adjacent diaphragm seating ring I6 or by adhesive means. A ring 21, preferably of the same radial` dimensions as ring 25, is positioned in coaxial relation to ring 25 and diaphragm projection 20 to spank the space between the diaphragm and thel sheet 2G. Ring 21 is made of suitable yieldable.

resilient material, and preferably has substantially the same properties as ring 25, .Thus a closed chamber is defined by sheet 26, ring 21 and the diaphragm, and any material 28 suitable to balance the pressure and vibration-damping aci tion applied at the opposite face of the diaphragm is confined in said chamber. A large variety of materials will satisfy the requirements for a balanced diaphragm. I prefer to use a granular material, and nd that carbon granules, sand, or granules of any other material which are of approximately the same size and specic gravity as carbon granules will serve the purpose. It will be understood, however, that such materials are mentioned .as illustrative only, and I do not limit.

the acceptable materials thereto, since any material which will equalize the pressure and vibraform of vibrations of sound of a given frequency;A and the vibration of the diaphragm'-responsive.

thereto, it will be apparent that application of similar conditions of pressureand vibration*- damping at the opposite diaphragm faces* will result in true uniformity of the wave forms of the diaphragm and the transmitted: signal oni each side of the median line represented by the normal non-vibrating position of the diaphragm; Hence distortion is avoided, and any damping action upon thediaphragmisequal and oppositeinY charactergandv does-notinterfere with true clell ityresponse ofthe-'diaphragmtolthe sound 'or signal applied thereto. Damping action -on the diaphragm,=' to-avoid undesirable` effects;- should beflocalized" and applied fat one point or portion taining ringgencircling saidbutt'on and'fspannngf the sp'acerbetween 'said diaphragm and one -housf ing wall," carbonv granules in theffring-*enclosed sDaceloetween"and engaging said diaphragm'projection and button, aseconld'retainer yringaligned with saidirst ring'gand*spanningithe space loe-L twee'n .sai'd'l diaphragm and the other" housing` wall; and granular material yconiinedin said last named ring and engaging sai'ddiaphragm withinthe cup-shaped portion thereof to dampenfsaid diaphragm" against harmonic distortion.

2. Av microphone comprisingfa-hollovf housing havingan opening'in one Yiizal-l'thereof, a diaphragxn` spanning said housing and having a central cupshapedfportion,' acup-shapedl button in said housing `spaced'fr'orn said-diaphragm and'adjacent said diaphragm projection, a yre-v taining ringvencircling said button and spanning the' space between said I diaphragm and `"one housingwall, carbonI granules" in the"V ring-enclosed space 'between and engagingisaiddiaphragm and button; a second retainer ring aligned withsaid first ring andspanningfth'e space between said diaphragmv andthe other housing wall, anda thin sheet of flexible material spanning" said opening, and'granular-'material within said-lastl named' fring, between said sheet and diaphragm and"V within' said cup-shaped r' projection for preventing harmonic' distortion of' 'said diaphragm. 3: A microphonel comprising a chambered housing;v adi'aphragm spanning f saidchamber andrhavinga" central offiset, a recessed'metal plate mountedin' said'housinginV spaced coaxial relation'tosaid diaphragm,-apairfof similar annular *retainers arranged "in" coaxial relation to each other and to said diaphragm and spanning the spaces between said diaphragm and the opposite walls of said housing, carbon granules confined by one ring and engaging said diaphragm end plate, and granular material confined by the other ring and engaging said diaphragm to counterbalance the pressure of said carbon granules on said diaphragm.

4.. A microphone comprisingI a hollow fhousing, a diaphragm in said housing dening opposed chambers therein, said diaphragm having a central cup-shaped portion, a contact in one chamber'in spaced 'coaxial relation to said diaphragm, a pair of similar retainer rings arranged in coaxial'relation to said-diaphragm and each spanninga chamber, carbon granules conned by one ring and engaging said diaphragm and plate, and-pressure equalizing means in the opposite chamber bearing against the center of said diaphragmV to prevent harmonic distortion of said diaphragm.

5: In @microphone iof thesingle carbonbutton type,fa1.housing=fhaving 'a centra-l off-setporg tion, a diaphragm in said housing. a contact;button in said housin`g,.carbon granulestbetweenand engaging saidbuttonand vafportion of onevfaceof said 'diaphragmirat said' offeset. and 4'means engaging.- av portion of 'theothenfa'cez ofssaidrsdiaphragm at `sai'doiifI-set in"pressurebalancing relation to said granules.v

6. In fa microphoneta hollow housing, afidiaphragm in said housing,v said diaphragm -having a central oisetasingle contact-.plate insaid housing, andy a` quantity of granularV materiall bearing against limited opposed-portions of eachi of the two faces of saiddiaphragm to balance said diaphragm, one quantity thereof vengaging said plate -and being electrically conductive.

7. In a microphone, a"h'ollow-h'ousing,v adia phragm in said housing, said diaphragm' having a central offset a single contactfmemberfinsaid housing spaced-from said diaphragm', aquantity of conductivex granular material between and yen-v gaging said contact memberand a-'-limitecl' porY against said contact and al limited'portion ofi one face of said diaphragm, and-means bearing against a limited'registering portion of the other `face ofsaid diaphragm, said granulesand means applying substantially equal pressures and vibration-damping factors to said diaphragm.

LOUIS R. BURROUGHS.

REFERENCESt CITED.'

The followingreferences are .0i record in the' lefV of this 1 patent:

UNITED STATES PATENTS:

Number Name Dater 1,033,087 Comer July 23,1912 1,073,667 Egerton Sept. 25,' 1913 1,079,931. Comer Dec. 2, 1913 1,509,818 Owen et al. Sept.l 23, 1924 1,546,749 Roberts Julyv21, 1925 

